This invention relates to surface coatings, and, more particularly, to a surface coating containing coated flaked metallic particles.
Surface coatings are often applied to substrates to protect the substrates. A familiar type of surface coating is a paint, which typically is applied to protect the substrate from corrosion or other types of damage, and/or to improve its appearance.
In another application, a surface coating may also protect the substrate from excessive heating, thereby aiding in the controlling of the temperature of the substrate. In one example, flaked metallic particles are added to the binders of surface coatings to act as heat reflectors. The metallic particles reflect incident radiation, particularly infrared radiation, preventing the radiation from reaching the substrate and heating it.
The metallic particles serve their heat-reflective function most effectively when their surfaces remain bright and shiny, that is, have a high surface reflectance in the wavelengths of interest. If the surfaces of the metallic particles become dull during service, as a result of oxidation, corrosion, or other causes, their ability to reflect heat is reduced, and the surface coating becomes less effective in protecting the substrate from the incident radiation. The substrate is heated to a higher temperature than would otherwise be the case. Designers must take such possible dulling of the particle surfaces into account in the design of the protected articles.
Oxidation and/or corrosion of the surfaces of the metallic particles may be expected in some practical applications of interest. For example, metallic flake particles may be incorporated into thermal protective coatings used on components of aircraft engines. These components are typically heated to temperatures in excess of 1500xc2x0 F. during service and exposed to highly corrosive gas turbine exhaust gas. At such temperatures, oxygen and corrosive components of the exhaust gas diffuse through the binder of the coating to the particles and oxidize their surfaces, reducing their ability to reflect incident radiation. The result in a gradual deterioration in the protective capability of the coating. Such deterioration of the particles also results from conventional rusting at lower temperatures.
It has been known to add an excessive amount of the metallic flake particles to the coating with the hope that some will remain effective for longer periods of time, thereby prolonging the protective life of the coating. This approach has the drawback that the protective life of the coating is prolonged only slightly at best, and the other properties of the protective coating may be adversely affected.
There is a need for an improved approach for particles used in heat-reflective coatings that will minimize or avoid deterioration of the coating over extended periods of use. The present invention fulfills this need, and further provides related advantages.
This invention provides coated flaked metallic particles and a method for their production, and a coating containing the particles and a method for its production. The coating contains metallic particles that reflect incident radiation. The coating retains its reflective properties over extended periods of time, either at room or elevated temperatures. It does not require any change in the coating formulation, except to stabilize the metallic particles against deterioration due to oxidation, corrosion, or other environmental surface effects. The present approach therefore allows any otherwise- operable particle material to be used, and is not limiting of the particle types. It also allows the binder of the coating to be selected without regard for its ability to protect the particles, so that it may be optimized for other properties.
A coating system comprises a coating including a binder, and a plurality of metallic flake particles dispersed throughout the binder. The particles have a surface-protective applied layer thereon. The coating is preferably applied to a substrate.
A method for preparing a coating comprises the steps of providing a plurality of metallic flake particles having particle surfaces, providing a binder precursor, depositing a surface-protective applied layer on the particle surfaces to form protected particles, and thereafter mixing the protected particles with the binder precursor to form a coating mixture.
The particles may be of any operable metal including, for example, gold and gold alloys, silver and silver alloys, platinum and platinum alloys, nickel and nickel alloys, and iron and iron alloys such as iron-aluminum alloys and iron-cobalt-aluminum alloys. The binder precursor is a material that hardens, dries, or cures to form the binder of the coating, and may include both organic and inorganic binders. Examples of operable binders include urethanes, epoxies, alumino silicates, or other polymer or ceramic binders.
The surface-protective applied layer may be deposited onto the metallic flake particles by any operable approach, such as vapor deposition, physical vapor deposition, chemical vapor deposition, atomic layer epitaxy, sol-gel processing, electroplating, electroless plating, passivation/pickling chemical treatments, and aqueous or gas-phase chemical surface treatments. The preferred approach is precipitation from solution, as by a sol-gel process. In such a process, the material of the surface-protective applied layer is controllably precipitated from a solution containing a precursor of the surface-protective applied layer. A preferred material for the surface-protective applied layer is silica (SiO2), precipitated at elevated temperature from an alcohol solution of tetraethyl orthosilicate.
The surface-protective material is deposited as a thin layer onto the surfaces of the particles. It passivates and protects the particle surfaces from oxidation, corrosion, and other adverse environmental effects. The particles therefore remain highly reflective for extended periods during service. It is not necessary to add excessive amounts of the metallic particles to maintain the heat reflective performance over extended periods.
The surface-protective applied layer of the present approach is to be distinguished from other types of surface layers, such as native oxide layers. The protective layer of the present approach is applied to the surface of the metallic flake particles, as distinct from being formed by a chemical reaction of the material of the metallic flake particles, such as the thermal oxidation of the material of the metallic flake particles. The present surface-protective applied layer may bond to the surface of its metallic flake particle by a chemical bond.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.